1. Field of the Invention
The present invention relates to fluid storage tanks that employ both dynamic and isolation seals and, particularly, to fluid storage tanks in which the isolation seal ruptures during system actuation without causing damage to the dynamic seals.
2. Description of the Background Art
One of the main sources of fuel for the engines of rockets and missiles is propellant fuel which is in the form of a liquid. This propellant is typically a liquid or gel, possibly with suspended solids, and is stored in cylindrical storage tanks located within the body of the rocket or missile. A piston slidably disposed within the storage tank maintains the pressure of the liquid fuel in the tank storage as liquid propellant is forcibly expelled from the tank through an appropriately located orifice. Another fluid, and most typically a gas, causes the piston to slide within the tank, thereby pressurizing the propellant or, alternatively, expelling the liquid fuel from the tank.
Since the liquid propellant may have to remain in its storage tank for many years before it is used, steps must be taken to ensure that the liquid propellant does not seep past the piston. Seepage of liquid propellant from the storage chamber of the tank into other portions of the rocket results in a loss of propellant and, thus, reduces the range of the rocket. Moreover, seeping propellant may allow propellant mixing causing ignition of the propellant outside of the combustion resulting in explosion or damage to other rocket propulsion components due to long term exposure because the propellant is extremely caustic.
A variety of design features have been employed to prevent the leakage of propellant as the piston moves within the storage tank. For example, elastomeric sealing rings are typically used to provide a dynamic seal between the piston surface and the interior surface of the storage tank as the piston moves within the storage tank. Disadvantageously, the materials from which such elastomeric seals are made can degrade rapidly while exposed to the liquid propellant. In time, proper operation of the piston and the rocket may be jeopardized.
In U.S. Pat. No. 5,042,365, there is disclosed an arrangement in which an annular shear seal is mounted within a storage tank about the outer circumference of the piston so that it also contacts the inner surface of the wall of the storage tank. The annular shear seal is structurally sealed, such as by welding or other bonding means, along its central opening to the outside surface of the piston and along its outer circumference to the inner surface of the wall of the storage tank. The annular shear seal is disclosed as having a notch portion which is positioned on the shear seal between the point where the shear seal contacts the piston and the point where the shear seal contacts the storage tank wall. The shear seal and its two welds block leakage of the propellant before the rocket is activated. When the rocket is to be activated, fluid pressure is applied to the piston, which causes the piston to exert an increasing force to the shear seal until a point when the shear seal ruptures along its notch. When the shear seal ruptures, the piston is free to slide and thereby expel the liquid fuel from the storage chamber of the tank.
Although the disclosed shear seal design aids in preventing leakage of the propellant, the design requires at least two separate welds--one between the piston and the shear seal, and the other between the shear seal and the wall of the storage tank. Since the integrity of these two welds is critical to the proper operation of the storage tank, both of the welds must be inspected prior to final assembly of the tanks. The inspection of these two welds is a relatively time consuming and expensive process in view of the fact that they are located within the narrow confines of the storage rank and must be carried out by using X-ray photography.
Additionally, a shear seal typically does not rupture uniformly along the entire length of its notch due to variations of the notch profile, variations in the depth of the notch, variations in the thickness of the surrounding material, and variations in the metal comprising the shear seal itself and local geometric distortions which are created by elongation of the shear seal material prior to rupture of the seal. As a result, it may be expected that a notched shear seal as disclosed above will not break uniformly, at exactly the same position, and at the same moment along its entire length. Furthermore, for the elastomeric seals to operate properly (i.e., provide a fluid tight seal while not impinging upon the freedom of movement of the piston), the rupture point of a shear seal cannot be recessed too deeply into the wall of the storage tank because the elastomeric seals which have to pass over the recess in which the shear seal is mounted cannot lose wall contact if sealing integrity is to be maintained. As a result, jagged remnants of the shear seal may be expected to remain, protrude, and have been found to tear or score the elastomeric annular seals on the piston. Such tears or scoring of the elastomeric seals allow the propellant to leak around the piston when the piston is activated, thereby possibly causing the piston to malfunction. Remnants of the shear seal may also cause the piston to jam so that all of the propellant cannot be expelled.
U.S. Pat. No. 3,545,343 discloses a design which employs only one weld rather than two or more welds, however, this weld is very deep. In this design, a weld is applied from the outside of the storage tank and passes through the entire wall of the storage tank into a recess in the piston. The weld is disclosed to be thin enough so that it will rupture or shear when the piston begins to move. This design poses problems relating to the formation of a uniform deep weld, and to the evaluation of the integrity of the weld because of its depth. In addition, the design does not protect the elastomeric seals on the piston from getting tom or scored by remnants of the weld after the weld has ruptured.
In an effort to address the above described deficiencies, the inventor herein disclosed in U.S. Pat. No. 5,385,081 assigned to the assignee of the present application, a shear seal and piston arrangement in which the head of the piston, in particular a circumferential shoulder on the outer periphery of the head of the piston, flattens against the wall of the fluid storage tank any protruding portions of the ruptured shear seal, including the jagged edge of the remnant of the shear seal that remains attached to the wall of the storage tank. As a result, the elastomeric seals are not damaged when the piston slides and remnants of the ruptured shear seal will not cause the piston to jam while it slides within the tank.
Although the configuration described in U.S. Pat. No. 5,385,081 does provide substantial protection of the elastomeric seals during movement of the piston, there is still a possibility that the seals may be damaged. The shear seal, which is bonded to the surface of piston proximate the head, is attached to the inner surface of the wall of the storage tank by a single weld located within a recess in the inner surface of the wall of the storage tank. As such, there exists a significant possibility that the elastomeric seals may be damaged or abraded as they pass over the area of the weld.